This invention is concerned with a process for the selective ortho-alkylation of a phenolic compound by catalytically reacting the phenolic compound with an alcohol in the vapor phase.
The preparation of 2,6-dimethylphenol, among other ortho-alkylated phenols, has heretofore been the subject of many studies, because it is useful as a raw material for the manufacturing of polyphenylene oxide having a wide range of utility in the fields of synthetic resins.
Currently, a process for the ortho-alkylation of phenols in a industrial use involves the vapor phase reaction of a phenol with an alcohol in the presence of a solid acid catalyst such as alumina. However, in this process, the selectivity for the ortho-alkylation is insufficient. That is, the meta and para positions of the phenol as well as the ortho positions thereof are alkylated to a considerable extent, so that a complicated procedure for the separation and purification of the ortho-alkylated product is required.
Another industrial process is based on the use of a magnesium oxide catalyst. But, this catalyst has inherently low activity, so that it requires reaction temperatures higher than 475.degree. C., practically higher than 500.degree. C., to perform the reaction sufficiently. Moreover, the life of the catalyst is not long enough, so the regeneration is required in a relatively short period of time for practical use.
In order to solve these problems, there have been proposed many kinds of catalysts; for example, those comprising various combinations of magnesium oxide and other oxides, or those comprising various combinations of iron oxide and other components.
These catalyst combinations, however, still have similar disadvantages those mentioned above, namely either they are insufficient in selectivity for ortho-alkylation and in service life or they induce the alkylation of undesired positions (other than the ortho positions) and the formation of polyalkylated products under such reaction conditions as to produce a satisfactorily high catalytic activity.
Meanwhile, a process for the selective ortho-alkylation of a phenolic compound having at least one ortho-positioned hydrogen atom by using a maganese oxide catalyst is disclosed in U.S. Pat. No. 3,971,832. The invention disclosed therein relates to a process for the ortho-methylation of a phenol which comprises contacting the vaporized phenol with an alcohol at a temperature of from 250.degree. C. to 500.degree. C. in the presence of a catalyst consisting essentially of trimanganese tetroxide previously calcined at a temperature of from 950.degree. C. to 1,500.degree. C. It is stated that one feature of the invention is to provide a process for the ortho-methylation of phenols with a very high degree of selectivity under mild reaction conditions. However, this process does not succeed in providing sufficiently high degrees of conversion of the phenol and selectivity for 0-cresol or 2,6-xylenol. Moreover, in order to obtain a high catalytic activity in this process, a manganese compound must be converted to trimanganese tetroxide by preheating it to a temperature higher than 950.degree. C.
The present inventors made extensive studies for the purpose of overcoming the difficulties encountered in the aforesaid prior art process, and discovered that a mixed oxide catalyst of manganese and silicon is effective for the selective ortho-alkylation of phenols and that the activity of this catalyst is much prolonged as compared with prior art catalysts. The invention based on this discovery is described and claimed in Japanese Patent Application No. 97874/77, which relates to a process for ortho-methylating a phenol by reacting the phenol with methyl alcohol in the presence of a mixed oxide catalyst of manganese and silicon.